In resonant tunneling diodes, the valley current is a leakage current that degrades performance. It is believed that the valley or leakage current has several origins, such as conduction through defects in the barrier, phonon assisted tunneling, thermally excited tunneling, hole conduction and others. Generally, a resonant tunneling diode, of the double barrier type, includes an injection layer, a first tunneling barrier layer, a quantum well, a second tunneling barrier layer and a collector layer all sandwiched together in the listed order. In an InAs-AlSb-GaSb-AlSb-InAs double barrier diode, for example, the electron confinement is good while hole confinement is poor.
With bias applied to the double barrier diode, normal current flows in the following manner. When the bias is sufficient to approximately align the ground state for holes in the quantum well with electrons in the conduction band of the injection layer, resonant tunneling occurs. That is, resonant tunneling occurs when one of the electron bands in the injection layer has the same energy as the ground state for holes in the quantum well. Then an electron from the filled ground state can transfer to the collector layer and an electron from the injection layer can replace the transferred electron.
Valley current occurs if the hole that is created in the quantum well moves to the negative terminal, the injection layer in this example. Further, as the bias increases, the barrier to holes decreases in height and thickness and valley current increases even more.